Atmospheric Oxidation of Coal at Moderate Temperatures Effect of Oxidation on the Carbonizing Properties of Representative Coking Coals L. D. SCHMIDT, J. L. ELDER, AND J. D. DAVIS Central Experiment Station, U. S. Bureau of Mines, Pittsburgh, Penna.
Preoxidized samples representative of eight different coal beds were carbonized to measure the effect of oxidation on yields and properties of carbonization products. In general, progressive oxidation results i n decreased pore size of coke, followed by poor fusion and finally by complete loss of coking power. The apparent specific gravity of coke increases with oxidation of the coal, but in general this change is accompanied by decreases both in strength of coke and in yield of by-products (except ammonia). The following proved to be sensitive measures of the extent of oxidation: yield of tar, physical properties of coke, especially when made in large re-
I
torts (80 kg. of coal), and agglutinating value. The majority of the coals tested show serious impairment of coking power before they are oxidized enough to decrease the heating value by 1 per cent and before their ultimate and proximate analyses show changes definitely greater than the experimental error of analysis. The amount of oxygen required to cause a decrease of 20 per cent in hardness of coke shows a threefold range between the most sensitive and the least sensitive coal tested. This oxygen requirement increases with the volatile matter and decreases with the oxygen content of fresh coals in a manner described by a quantitative relationship.
x GENERAL, when crushed coking coals are heated above
able to know the probable effects of adventitious preoxidation so that special precautions may be taken with the more susceptible coals. The susceptibility to change in carbonizing properties on exposure to air depends primarily on two factors: (a) the characteristic rate of oxidation of the coal-that is, the rate of conspmption of oxygen under a given set of conditionsand (6) the effect of a given amount of oxygen consumption on the carbonizing properties. The first paper of this series (9) dealt with the characteristic rates of oxidation of the coals tested. This paper will discuss only the effects of oxidation on the carbonizing properties of the same coals. The apparatus and procedure used to prepare samples of each coal a t various stages of oxidation were described in the first paper
their decomposition temperatures, the original coal particles fuse into coherent blocks of coke in which the identifying outlines of the original pieces of coal are completely lost. Storage in air causes deterioration in this ability to fuse and also results in changes in other carbonizing properties, often long before any change in proximate and ultimate analysis can be detected. The present investigation was undertaken by the Bureau of Mines to study the effects of preoxidation on various carbonizing properties and to measure for several commercially important coking coals their susceptibility to change on storage. Susceptibility to change in carbonizing properties on storage varies greatly with coals from different beds and is an important factor in determining, for example, how much coal should be stored in coke-oven operation where large quantities are desirable to provide for seasonable variations in supply changes in mix to give certain coking properties on demand, and interruptions in supply of coal. I n a few plants deliberate preoxidation of coal on a commercial scale is practiced to attain desired changes in carbonizing properties-for example, a reduction in “stickiness” (4, 6). Preoxidation of coal is also reported to aid in the production of activated carbon (6). I n the laboratory the controlled regular changes in carbonizing properties that can be secured by oxidation are valuable in throwing light on some of the complex phenomena incident to the coking of coal. For example, it is of assistance in correlating laboratory measurements of coal plasticity with the physical characteristics of coke ( 3 ) . In the preparation and care of laboratory samples of coal it is highly desir-
(9)‘
The source of each coal tested is given in the following table; throughout the tables of this paper’ the coals are listed in the order of increasing rank ( 1 ) . Coal Mine Place Closplint Closplint Bruceton Bruceton Majestic Majestic Keen Hangar Mountain Wyoming Wyoming 55 Sewell 48 Lower Kittanning . . . . . Johnstown (washed) 56 Pocahontas No. 3 Buckeye Stephanson No. 3 57 Pocahontas N o . 4 KO.4 Affinity
NO.
54 52 53 58
548
Bed High Splint Pittsburgh PondCreek Lower Banner
County Harlan Allegheny Pike Buchanan
State Ky. Penna. KY. Va.
Wyoming W. Va. Cambria Penna. Wyoming W. Va. Raleigh
W. Va.
INDUSTRIAL AND ENGINEERING CHElTISTRY
APRIL, 1940
Carbonization Procedure The Bureau of Mines-American Gas Association (BMAGA) method ( 2 ) for determining the gas-, coke-, and byproduct-making properties of coal was applied to the various oxidized samples. Samples of crushed coal (80 kg.) were carbonized in cylindrical mild-steel retorts (diameter, 46 em.) in a furnace maintained a t 800" C. The physical and chemical properties of the products mere determined by standard methods. Some coals were carbonized a t 900" C. in a smaller retort ( 2 kg. of coal) with a by-product recovery train that has been described (IO). 40
36 s W
I
0
1
,
-
,
2
3
4
5
6
OXYGEN CONSUMED, PERCENT BY WEIGHT MOISTURE AND MINERAL MATTER FREE COAL
FIGCRE1. VARIATIONIN VOLATILE MATTER CONTENT KITH OXYGEN CON~UMED BY THE COAL
The reason for selecting the two furnace temperatures of 800" and 900" C. was that, in general, with fresh coals these temperatures produced the best coke-that is, coke comparing favorably with that from commercial installations. To conserve space the mass of experimental data obtained must lie presented in brief form; in many instances only the general variations in carbonizing properties with oxidation are given.
Effect of Oxidation o n Coal Analysis The chemical and petrographic analyses of the fresh coals were given previously (9). Ultimate and proximate analyses were made on each sample of coal a t its various states of oxidation. It will be advantageous to discuss briefly the changes in TABLEI. EFFECTOF analvsis caused bv oxidation Oxidabefore changes in carbonizing Coal Volatile tion pro pert ies a r e considered. No. Matter Temp. %a C. These changes in analysis will 99.3 54 39.6 be represented by empirical 52 39.3 99.3 e q u a t i o n s mh er e possible. 52 38.9 50.0 52 39.8 30.0 Heating value, carbon content, 99.3 53 34.3 and oxygen content all changed 5s 23.1 99.3 55 22.5 99.3 linearly with the extent of oxi50 18.3 99.3 dation of the coals. 57 18.0 99.3 The changes can be expressed Average 9 9 . 3 as follcw:
(C) = yo carbon as shown by ultimate analysis (ash- and moisture-free basis) X = amount of preoxidation = oxygen consumed, yo by weight of dry, mineral-matter-free coal If,, (C)o, (0)05 e, g, k = constants, the values of which are shown in Table I for each coal tested. (For Pittsburgh coal, No. 52, for which the greatest number of determinations were available, the average percentage deviation between observed and calculated values was as follows: .Equation 1, 0.1; Equation 2, 0.2; Equation 3, 2.6.:)
The percentage of ash, nitrogen, and sulfur in the coals did not change appreciably with the amount of oxygen consumed in these tests. The percentage of hydrogen showed a slight tendency to decrease. The volatile matter content changed in an irregular manner with oxidation, as Figure 1 shows. Many of the curves exhibit a minimum value followed by a rather indefinite increase in volatile matter content with further oxidation. Study of the data on change in analysis with oxidation leads to several general conclusions. The observed percentage decrease in carbon content is due primarily to increasing weight of coal as oxygen is added. The average value of X: for all coals oxidized a t 99.3" C. indicates that about half of the total oxygen consumed remains in the coal. The decreases in heating value observed for Pittsburgh coal oxidized a t 30 O , SO", and 99.3 " C. indicate that in this temperature range the effect on heating value of a given amount of oxygen consumed is essentially independent of the temperature of oxidation. Among the various values found by proximate and ultimate analysis of coal, heating value is probably the most reliable measure of the extent of oxidation. I n a previous investigation by the Bureau of ,Mines ( 7 ) coal from the Sewell bed in Fayette County, W. Va., was crushed to pass a 6.4-mm. (0.25-inch) screen and then stored for 5 years in small piles (23 and 160 kg.) a t various points in this country. Periodic sampling and analysis showed a small but fairly regular decrease in heating value, which can be expressed by the following equation (the average percentage deviation between observed results and those calculated by means of this equation is 11.4) :
looaH = 0.0106T0.67= % decrease in heating value (ash- and HO moisture-free basis) after 2' days in storage According to this relation Sewell bed coal stored under the conditions prevailing in these tests (average air temperature, 12.5" C.) mould show a 1 per cent loss in heating value after 887 days (2.4 years) of storage. I n the laboratory test a t
Time
Heating Value
T
ffo
6.6 4.5
Cal./g. 8244 8387 8403 8459 8528 8689 8697 8711 8694
...
8563
DaVsb 2.2 3.7 53.3
356.0 4.3 4.6 5.3
Constant e Cal./g.c 30 68 71 71
l%?
HO
(Cjo
Constant f Q
(010
Constanth h.
%"
%" 83.6 84.1 84.6 84.7 86.4 SO. 1 89 9 91.3 90.4
0.29 0.41 0.57 0.96
8.7 7.0 6.3 G.6
0.62 0.37 0.22 0.72 0.20
5.g
52
0.47 0.81 0.84 0.84 0.78 0.62 0.75 0.72 0.GO
2.r 3.0 2.3 2.6
0.7 0.6 0.4 0.6 0.3
58
0.68
87.8
0.40
5.0
0.4:
!i
04 66 62
%,a
0.3 0.5 0.6 0.7
Moisture- and mineral-matter-free. h Days of oxidation in air required for coal t o consume 1 per cent of its !\-right in oxygen (moisture- and mineralmatter-free basis). e Decrease in heating value for 1 per cent oxygen consumed. d Decrease in heating value for 1 per cent oxygen consumed. e Carbon content of fresh coal. f Decrease i n percentage carbon for 1 per cent oxygen consurneil. 9 Oxygen content of fresh coal. h Increase in percentage oxygen in coal for 1 per cent oxygen consumed. a
where H = gross heating value, cal./gram of ash- and moisturefree coal
349
INDUSTRIAL AND ENGINEERING CHEMISTRY
550
TABLE11. EFFECTO F
O X I D 4 T I O N O F C O i L ON CiRBONIZATION YIELDS I U D P H Y S I C i L PROPERTIES O F C O K E FORMED
Carbonization Yieldsc, Coal NO.
X. %"
VOL. 32, NO. 4
Ta,Daj-sa
Coke
0 0 0.9 2.2 5.3 7.0
67.5 1 1.001 1.001 1.004 1.006
7.5 1 0.973 0.933 0.867 0.827
17.9 1 1.006 1.011 1.022 1,028
0
65.1 1 1,009 1.017 1.029 1.037 71.2 1 1,004 1.008 1.017 79.5 1 1 1 1 1
9.7 1 0.938 0.866 0.732 0.598
80.7 1 1.001 1,002 1.005
8.1 1 0.914 0.827 0.667 3.9 1 0.923 0.872 0.718 0.667 4.2 1 0.905 0,810 0.619
16.2 1 1.006 1,012 1,031 1.049 14.2 1 1.021 1.042 1,085 12.8 1 1,008 1.016 1.031 1.039 11.6 1 1.017 1.034 1.069
84.7 1 0.995 0.995 0.998 84.6 1 0.998 0.994 0.996
2.4 1 0.875 0.792 0.667 2.0 1 0,900 0.800 0.750
10.9 1 1,009 1.009 1,018 11.3 1 1 1 0.991
Tar
Gas
Liquor
'iight oil
Total SH3
'
' Amount
fused5
Physical Properties of CokeApparent HardShatter sp. gr. Stability! nessg Friabilityh indexi
Coal Oxidized at 99.3O C.. Carbonized a t 800' C.: BAl-AGA Procedure 54
0
0
0.5 1.0 2.0 2.5 52
53
58
65
0 0 0.5 1.0 2.0 3.0 0 0
0.5 1.0 2.0 0 0 0.5 1.0 2.0 2.5 0 0
0.5 1.0 56
57
2.0 0
0 0.5 1.0 1.5 0 0
0.5 1.0 1.5 54
2
53
52
48
52
0 0 2.8 6.6 11.0 0 0 1.9 4.5 7.5
0
0 0 0.5 1.0 2.0 2.5 0 0 0.5 1.0 2.0 3.0 4.0 5.0 0 0 0.5 1.0 2.0
0 0.9 2.2 5.3 7.0 0 0 1.5 3.7 8.7 14.5 20.8 27.5 0 0 1.8 4.3 10.2
0
0
0.5 1.0 2.0 3.0 4.0 5.0 6.0
0
0
0 0.5 1.0
0 0 0.5
1.0 0
0
1.5 3.7 8.7 14.5 0 0 1.8 4.3 10.2 0 0 1.9 4.6 11.0 14.5 0 0 2.2 5.3 12.6
65.3 1 1.003 1.008 1.014 1,017 64.9 1 1,009 1.017 1,029 1.037 1.040 1.042 69.6 1 1.004 1.007 1.014
4.6 1 1 1 1 1,022 4.9 1 1.041 1.061 1.122 1,204 4.5 1 1.022 1.067 1.133 2.5 1 1.040 1,120 1.240 1.320 2.3 1 1.043 1.130 1.217 1.4 1 1.214 1.429 1.571 1.3 1 1.308 1.692 2,000
1.2 1 1 0.92 0.83 0.83
0.408 1 1.017 1.034 1.069 1.088
99.5 1 0,905 0.799 0.568 0.397
0.79 1 1.025 1.051
1.0 1 1 1 1 1 0.9 1 1 1 1 0.6 1 1 1 1 1 0.6 1 1 1 1
0,285 1 1.025 1.053 1.168
...
97.4 1 1.002 1,003 1.004 1,002
0.83 1 1 1 1 1
0,292 1 1.031 1.062 1.127 0.316 1 0.987 0.975 0.949 0.937 0.254 1 1.047 1.094 1.193 0.219 1 1.023 1.041 1.068 0.278 1 1.004 1.007 1,011
99.2 1 0.998 0.996 0.982 99.5 1 1 0.995 0.804 0.518 100.0 1 1 0.995 0.750 98.7 1 0.976 0.811 0.253 98.0 1 0.918 0.699 0.301
0.82 1 1,012 1,037 1.073 0.94 1 1.011 1.011 1,021 1.021 0.87 1 1.023 1.034 1.069
0.5 1
1 0.80
0.80 0.5
1 1 1
0.80
,..
...
0.87 1 1.011 1.011 1.023 0.85 1 1.024 1.035
...
Coal Oxidized a t 99.3' C., Carbonized a t 900' C.; 2-Kg. Apparatus 10.2 19.3 5.: 99.5 0.79 I
0.961 0.922 0.843 0.804 12.1
I
I
1.010 1.016 1.036 1.047 17.3
1 1.020 1.020 1.020 4.7
16.9 1 1 1 1
1.043 1.064 1.128 1,191 1.255 1.319 2.7 1 1.370 1.556 1.630
I
10.6 1 0.934 0.868 0.736
.. ..
.. .. .. .. .. .. .. ....
.. .. .. .. ..
...
... ... ... ... ... ...
... ... ...
... ...
... ... ... ... ...
1
I
0.995 0.990 0.963 0.935 98.4
1.038 1.063 1.051 1.025 0.78
I
1,001 1,002 1 0.996 0.983 0.958 98.2 1
1.005 1.008 1
0.79 1 1.038 1.063 1.114
Coal Oxidized a t 50' C., Carbonized a t 900' C.; 2-Kg. Apparatus 13.2 16.8 4.3 97.8 0.78
.. .. .,. ... .. .. ..
... ... ...
22.4 53.3 127 211 302 399 501 0 0 23.3 55.3
64.9 1 1.008 1.015 1,025 1,028 1,028 1,029 1.029 82.5 1 1.001 1.001
0.932 0.871 0.773 0.682 0.644 0.621 0.614 3.0 1 0,833 0.667
0 0 127 356
63.8 1 1.013 1.027
Coal Oxidized a t 30' C., Carbonized a t 900° C.; 2-Kg. Apparatus 4.8 ... 97.3 14.0 16.6 1 1 1 1 0.958 ... 0.999 0,929 1 0.917 ... 0.999 0.857 1
0
I
I
1.006 1.018 1.030 1.041 1.053 1.071 1.089 12.9 1 1.016 1.039
I
047 116 ,209 ,326 ,442 ,535 ,651 1.1 1 1.364 1.818
.. .. ...... .. ......
... ...
... ...
...
...
... ...
... ...
98.7 1 0.939 0.831
...
0.84 1 1.012 1.024
.... .. ...
...
72.1 1 1.033 1.064 1,130 1.165 68.7 1 0.990 0.990 1.044 1.128
73.9 1 0.928 0.855 0.709 0.633
53.6 1 1.002 1.002 1.157 37.5 1 0.960 0.960 1.400 1.760
88.5 1 1.005 1.007 0.963 94.9 1 1.014 1.014 0.919 0.823
...
36.8 1 1.011 1.060 1.549 47.5 1 1 1.257 1.589 47.5 1 1 1.257 1.589
96.0 1 1.004 0.996 0.902 94.0 1 1.004 0.915 0.761 95.5 1 0.965 0.892 0.790
62.3 1 1 0.952 0.602 0.377 48.5 1 1.058 1.095 1.093 1 0,907 0.816 65.6 1 1.096 1.146 0.992
78.0 1 0.997 0.974 0.731 0.558 63.2 1 1.030 1.055 1.044 0.970 0.894 0.816 74.4 1 1.050 1.078 1.070
52.5 1 1.013 1.051 1.286 1.476 59.7 1 0.961 0.931 0.970 1.025 1.080 1.136 49.6 1 0.982 0.980 1.073
57.8 1 1.045 1.088 1.163 1.183 1.163 1,130 1.093 71.2 1 0.895 0.140
67.2 1 1.036 1.067 1.110 1.119 1.110 1.100 1.086 79.5 1 0.889 0.629
55.5 1 0.966 0.950 0.932 0.933 0.946 0.950 0.995 49.6 1 1.149 1.512
55.6 1 1.110 1.153
64.2 1 1.065 1.132
57.8 1 0.917 0.903
26.0 1 0,981 0.885 0.562 0.373
66.5 1 0.962 0,880 0.647 0.516
33.8 1 0.997 0.994 0.959 0.763 59.4 1 1,003 1.003 0.848 70.7 1 1.033 1.042 0.816 0.615 72.7 1 1.004 0,990 0.715 62.0 1 0.968 0.823 0.484 61.3 1 0.873 0.742
61.8 1 1,002 1.003 0.971 0.647 68.9 1 1.001 1,004 0.856 73.5 1 1.024 1,034 0.839 0.660 74.3
...
1
1,003 0.983 0.740 65.5 1 0,971 0,840 0.542 63.7 1 0.879 0.754
76.6 1 1,023 1.031 1.014 0.953
Oxygen consumed per cent b y weight of moisture- and mineral-matter-free coal.
b Required days oxibation of coal (0-6.4 mm.) i n air (20.93 per cent oxygen) a t temperature given. C
Basie, moisture- and ash-free coal.
d All values for oxidized coals expressed as ratios t o values for fresh coal ( X = 0). a 25-mm. (1-inch) screen in BM-AGA procedure; a 6.4-mm. (0.25-inch) screen in small-scale e Coke from retort, cumulative per cent retained on: procedure. / Cumulative per cent retained on: a 25-mm. (1-inch) screen a f t e r BM-AGA tumbler; a 19-mm. (0.75-inch) screen after small-scale tumbler. I Cumulative per cent retained on a 6.4-mm. (0.25-inch) screen after tumbler. h Per cent reduction in particle size of coke during tumbler test. i Cumulative per cent retained on a 38-mm. (1.5-inch) screen after BM-AGA shatter test.
APRIL, 1940
INDUSTRIAL AND ENGINEERING CIIEMISTRI
99.3' C. Sewell coal required 9.6 days to consume 1.33 per cent of its weight (dry, mineral-matter-free hasis) of oxygen, which corresponds to a 1 per cent decrease in heating value (Table I). Consequently, as pertains to loss in heating value, 1day of oxidation in the drum at 99.3" C. is equivalent to 92 days of storage of this coal a t atmospheric temperatures. An independent check on this result is obtained by calculatirig the relative rates of oxidation at 12.5O and 99.3' C. by means of the Arrhenius equation. IJsing the value 11,000 calories found previously (8) for the heat of activation (Yittshurgh coal) between 30" and 100" C., the ratio of the rates of oxidation at 99.3' and 12.5" C. is found to be 92. This happens to be exactly the figure obtained above for the relative rates of decrease in heating value under the different oxidizing conditions. For other coals and other storage conditions this equivalence ratio probably will be found useful only for rough estimates.
Carbonization Yields The results showing the effect of oxidation on yields of carbonization products and on the strength of coke are given i n Table IT. All the values were taken from smooth curves drawn through the actual experimental points. This method of presenting the data was chosen primarily to allow, comparison of different coals at equal states of oxidation, that is, equal values of X . All the results given in the table for oxidized coals are expressed as ratios to the corresponding values for unoxidized coal. The percentage change in any property with oxidation is the difference between the ratio and unity multiplied b y 100. The results obtained by carbonizing 80-kg. samples of coal that had been oxidized in air a t 99.3" C. will he discussed first
551
bocause the carbonizing coiiditions in these tests approach more closely those used in commercial installations. For all of the coals tested the yields of tar (column 5 ) decrease rapidly with oxidation; in fact, the decrease in tar yield is the most sensitive carbonizing property for indicating the extent of oxidation of a coal. For 1 per cent oxygen consumed, the average decrease in tar for all coals is 16 per cent of its average yield. Thr corresponding increase in coke is 0.3 per cent of its averagc yield. The relative increase in yield of coke is small compared to the decrease in tar; nevcrtlieless, because of the greater quantity of coke produced, the sum of the yields of tar and coke for each coal varies but sli&tly with the extent of oxidation. This fact indicates that oxidation of coal changes the original tar-forming constituents in such a way that upon carbonization they decompose to form coke and a small quantity of gas instead of distilling as tar. The yields of gas (column 6) increase slightly with oxidation of the coal. This increase is duc to increased evolution of the oxides of carbon, chiefly carbon dioxide. The yields of liquor (column 7) increase rapidly with oxidation of the coals. The increase is particularly high for those coals that normally show low yields of liquor-that is, high-rank coals. The yields of light oil (column 8) tend to decrease somewhat with oxidation. However, their detennination is not precise enough to show a regular decrease. Yields of ammonia (column 9) show a definite increase with oxidation of the coal (except for Lower Banner coal 58).
Physical Properties of Coke From a commercial viewpoint the most important change in carbonizing properties is the general deterioration in strength of coke caused by oxidation of the coal (Figure 2).
552
INDUSTRIAL AND ENGINEERING CHEMISTRY
I n Figure 2 the small triangular sections are photographs of typical pieces of coke as taken from the retort. These pieces of coke are formed in the retort with the apex pointed toward the center of the retort. For several coals a t the higher states of Oxidation, the crushed coal fused upon carbonization to form coherent coke only for a short distance from the heated wall of the cylindrical retort. In these instances the entire middle portion of the retort was filled with granular char or unfused particles of coke; as a consequence the pieces of coke are shortened or truncated as shown in the photographs.
VOL. 32, NO. 4
half retort) of about 3.3 cm. (1.3 in.) per hour. This compares with 1.3 cm. (0.5 in.) per hour in the large retort. The shape of the two retorts is the same, but the great difference in size makes for significant differences in distance of travel of tar and gas through hot coke. However, in general, the results in the two retorts compare favorably in many respects, perhaps because of mutually compensating factors. The results obtained in the small retort are included in Table 11. The yields of the various products do not differ greatly with the retort size. The yields of coke are consistently about 1.5 per cent (ash- and moisturefree basis) lower in the small retort but show the same percentage increase with oxidation. The yields of tar average about 2.5 per cent higher in the small retort but change with oxidation of the coal in approximately the same manner as in the large retort. The yields of gas are about 1.5 per cent higher in the small retort, but the effect of oxidation of the coal is about the same in both retorts. The yields of liquor do not differ sig0 04 08 12 16 20 24 28 32 36 40 44 48 52 56 nificantly. The yields of light oil and ammonia OXYGEN CONSUMED PERCENT BY WEIGHT MOISTURE AND MINERAL MATTER FREE COAL were not determined in the small retort. The carbonization loss (difference in weight between OF OXIDATIOX OF COALOK AGGLUTINATING VALUE FIGURE 3. EFFECT coal and the Droducts of carbonization)' is considerably greater in the large retort. The rectangular sections show the fine structure of the coke. The percentage fusion of the charge with the unoxidized (For each coal a t its various states of oxidation these sections coals tested is the same in both retorts; that is, virtually all are taken a t equal distances from the wall of the retort.) the charge is fused. However, when the coals are oxidized, They show that, in general, the first stage of oxidation results the percentage of fusion decreases much more rapidly in the in a decrease in size of pores or increased fineness of structure. large retort. This difference in behavior is due primarily to More oxidation results in poor fusion or pebbly coke in which the large difference in heating rates, the higher rates causing the outlines of the original coal particles can be seen. Shrinkage better fusion of oxidized coal. cracks are usually less pronounced, and the convex or %auliThe strength of coke made in the small retort was measured in a small tumbler (IO). Special tests were carried out on the flower" ends showless curvature in coke made from oxidizedcoal. same cokes in both the small and the large (BM-AGA) Table 11, column 10, shows quantitatively the increase in tumblers to compare results. These tests on cokes covering amount of unfused coke with oxidation. The values given a large range in physical properties showed that the followwere obtained from the screen analyses of the coke from the ing conversion factors could be used to place results obtained retorts. The percentage fused is taken arbitrarily as the cumulative percentage upon a 2.54-cm. (1-inch) screen. In in the two tumblers on a comparable basis: The stability in the small tumbler (on 19-mm. screen), hardness (on 6.4-mm. general, the amount of unfused coke remains small until, screen), and friability should be multiplied by 0.76, 0.92, with increasing oxidation of the coal, a critical value is and 1.42, respectively, to give values comparable with those reached ; then it increases rapidly with further oxidation. obtained in the larger tumbler. (Friability is the percentage For all coals tested, oxidation increased the apparent reduction in average particle size after 1 hour in the tumbler. specific gravity of the coke formed (column 11). Coke It is calculated from the complete screen analyses of cokes, strength as measured by tumbler and shatter tests (columns using the method of calculation of Y ancey and Zane, 11.) 12 to 15) shows the same tendency for slight change with If allowance is made for the different tumbler tests, the oxidation up to a certain critical value, and then the strength coke-strength results (Table 11) on unoxidized High Splint decreases rapidly with further oxidation. (54), Pittsburgh (52), and Pond Creek (53) coals as carEffect of oxidation on agglutinating value or crushing bonized in the small retort show nearly the same hardness as, strength of the coals is shown in Figure 3. (The ratio of and 15 per cent greater friability than in the large retort. silicon carbide to coal is 15 to 1 for all coals except 52, where The relative stability of coke made in the two retorts the ratio of sand to coal is 15 to 1.) For most coals the change varies greatly with the coal; on an average for the three coals in agglutinating value with oxidation parallels corresponding it is 11 per cent higher in the small retort. changes in physical properties of coke. The value of the The effect of oxidation on strength of coke differs conmethod as an indicator of the extent of oxidation of coal ( 8 ) siderably in the two retorts. The results obtained with High is confirmed by the rapid decreases observed for coals that are Splint coal (54) show that the strength (as measured by stasensitive t o oxidation. bility, hardness, and friability) of coke from the large retort Effect of Size of Retort on Carbonization Results decreases with less oxidation but approaches a similar percentage decrease when the coking power is greatly impaired High Splint (KO. 54), Pittsburgh (KO.52), and Pond Creek (X = 2.5). For Pittsburgh coal (52) carbonized in the small (No. 53) coals were carbonized a t various stages of oxidation retort, oxidation causes a decided increase in coke strength as in both the small (2 kg. of coal) retort a t 900" C. and in the measured by stability, hardness, and friability. The maxilarge (80 kg. of coal) retort a t 800" C. The results show the mum strength is reached when about 1 per cent oxygen has effect of changing carbonizing conditions. Probably the most been consumed ( X = 1). The results do not show so great an important difference between the two tests is in the heating increase in strength when the coal is carbonized in the large rate, which is much higher in the small retort. The average retort, although the friability and shatter tests do indicate time of carbonization in the small retort is about 115 minutes, increased strength of coke. which corresponds to an average over-all coking rate (one-
APRIL, 1940
INDUSTRIAL AND ENGIYEERING CHEMISTRY
The strength of coke from Pond Creek coal ( 5 3 ) , whether carbonized in the large or the small retort, increases with oxidation until a maximum is reached. This maximum is less pronounced and is reached Tvith less oxidation when the coal is carbonized in the large retort. In general, the results on the three coals show that the higher rate of heating obtained in the small retort tends to emphasize any improvement caused by mild oxidation and also to defer the ultimate general deterioration brought on by further oxidation. It was shown above (Table I and its discussion) that the temperature of oxidation has but little effect on the magnitude of the decrease in heating value of coal caused by a given amount of oxygen consumed. Examination of the carbonizing results in Table I1 for the effect of temperature of oxidation on Pittsburgh coal shows that, in general, it is a factor of secondary importance, a t least in the range 30' to 99.3' C. For example, 1 per cent oxygen consumed decreases the yield of tar by 14, 13, and 16 per cent, respectively, for oxidation a t 30', 50°, and 99.3' C. Since yields of tar decrease rapidly with oxidation of the coal, the differences between these values are not of great significance. Effect of O x i d a t i o n on Tar, Light Oil, and Gas Table I11 shows the effects of oxidation of the coals on the yields of some of the constituents of tar and light oil. The values for oxidized coals are expressed as ratios to the corresponding values for fresh coal (X = 0). Although the analyses of tar and light oil show only small changes with oxidation of the coals these changes deserve discussion because they may throw light on the points of attack of oxygen on coal. When the coals have each consumed 1 per cent by weight of oxygen the tar yield on an average for all coals decreases by 16 per cent. Contrary to what might be expected, the yield of tar acids decreases more rapidly with oxidation than the
TABLE111. EFFECT O F OXID.4TIOlv 7
Coal h-o.
S-
-
p
Total tar
52
53
58
55
56
0 0 1.0 2.0
Tar acids
8.52
0.92 0.85 86.4
0 0 1.0 2.0 3.0 0 0 1.0 2.0
0.86 0.71 0.57 72.2
1
1
0.83 0.65
0 0
35.5
1.0 2.0
0.84 0.69
0 0 1.0 2.0
36.3
0
0 1.0 1.5
57
65.5
1
0 0 1.0 1.5
1
1
0.81 0.62 20.4
1 0.80 0.70 17.1 1
0.84 0.76
Tar residue or pitch
-
R e l a t i v e S e n s i t i v i t y of Coals to O x i d a t i o n Table IV compares the various coals on the basis of the amount of oxygen (value of X) required to cause certain
O F c O . I L AT
99.3"
+
Total tar light oil
Total aromatics
c. ON T A R , LIGHT0 1L, AND GAS
0.94 0.88
0.!22 0.84
12.4 1 0.81 0.62 0.44 8.16
43 3
0.92 0.85 99.3
1
1
1
0.84 0.69 0.54
0.87 0.74 0.60
0.92 0.85 0.77
36.1
83.8 1 0.84 0.69 44.0
25.9
1
1
1
0.76 0.52
0.85 0.69
1.17
20.7
0.77 0.60 1.60
0.87 0.76 23.1
1
1
1
0.75 0.50 0.59 1
0.85 0.76 0.22 1 1.1s 1.36
1
0.84 0.67 13.2 1 0.85 0.77 11.1 1 0.84 0.76
Total paraffine naphthenee
+
-
79.6
1
Total olefins
p e r metric ton---h,a
31.0
a Of dry ash-free coal. b Liters per metric ton X 0.240 = gallons per ton (2000 Ib.)
Kg.-cal. per cubic meter X 0.105
yield of tar in coals 52, 53, 58, and 55. The yield of tar acids decreases with oxidation for all coals except 57, which shows such a small yield that experimental error may explain its anomalous trend. For the majority of the coals the yield of pitch decreases less rapidly with oxidation than the total yield of tar; that is, the tars made from oxidized coals tend to show slightly increased pitch content. The total yield of tar plus light oil (column 6) decreases with oxidation more slowly than the yield of tar alone. The yield of total aromatics in tar and light oil (column 7) tlecreases more slowly with the oxidation of each coal than the yield of tar plus light oil. This may indicate that the sources of aromatics in the coal are somewhat less subject t o attack by oxidation than the sources of other constituents in the tar and light oil. The yield of total olefins (column 8) decreases a t about the same rate as the yield of tar and light oil, whereas the yield of paraffins and naphthenes (column 9) shows for most coals a considerably faster rate of decrease. Saphthalene and anthracene yield increases rapidly with oxidation of coals 52, 53, 58, and 55, all of which show low yields of these constituents for fresh coal. For coals 54, 56, and 57, where the yield of naphthalene and anthracene is high from fresh coal, the yield decreases with oxidation of the coal. The heating value (column 11) of the gas evolved on carbonization decreases with oxidation of each coal tested. Since the only significant changes in gas analysis (not shown) are the increasing carbon dioxide and monoxide content, the observed decrease in heating value must be caused primarily by the diluent effect of these gases. The calorific yield (calories in the gas per gram of dry, ash-free coal) decreases with oxidation of all coals except 53 and 58 (column 12).
Yield of T a r , Light Oil, and Their Constituents
-Liters 54
553
1
1
0.87 0.74
43.5
28.5
4.73 1
1
0.92 0.85 30.9
0.95 0.89
0.91 0.82 3.95
1
1
0.89 0.79 15.1
1
0.90 0.79 11.7
1
1
0.83 0.66 25.5
0.84 0.69 7.69
5,63 I
0.84 0.68 0.52 4.12 1 0.82 0.63 2.47 1
3.20
I
0.83 0.67 0.50 6.70 1
Calorific yield
1
1.31 1.60 1.88 0.052
1
1
1.46 1.95 0.028
1
cu.
m.C
6070
Cal./g.a
1
1860 1
0.97 0.93
0.9s 0.96
6160 1
1790
0.97 0.93 0.90 6070
0.98 0.97 0.95 1710
1
1
1
0.98 0.97 5750
1.01 1.03 1650
1
1 1 1
0.9s 0.97 5580 1
1590 1
1.79 2.86
0.97 0.94
0.98 0.96
0.99 1
0.14
1
0.50 0.21 0.18
5170 1 0.97 0.96 6170
0.99 0.97 1480
0.97 0.96
0.99 0.97
1
1
0.85 0.78 22.8
0.86 0.79 7.32
1
1
0.85 0.75 0.93 1
1
0.86 0.78
0.89 0.83
0.97 0.96
0.90 0.89
B. t. u. per cu. ft. (saturated a t 60' F. and 30 in. Hg).
0.042
0.041
1
3.23 1 0.71 0.40
1
1 0.85 0.65
2.71
1.76
0.94
wt."
0.10
2.17 3.40
0.70 0.41
1
Yoby
7----GasF Heating value
KO.-cal../
0.64 0.28
0.81 0.65 0.85 0.74
+Naphthalene Anthracene in T a r
1 0.88
1
1
0.61 0.39
1
1480 1
1
554
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE Iv. Coal
No.
99.3" c. REQUIRED TO GIVE CERTAIN CHaNGES I S COKE STRENGTH AND VALUES OF CARBONIZING PROPERTIES (BM-AGA PROCEDURE)
h l O U N T O F OXIDATION AT
S
TO
Amount Cnfused
%
Days
%
54 52 53 58 55 56 57
1.4 2.55 2.22 2.12 1.84 1.1 0.82
3.4 11.8 11.5 11.6 11.3 7.5 3.6
a+
0.66
1.3
14
53 58 55 56 57
2136 2.14 2.04 1.04 0.96
l2:5 11.9 12.9 6.97 4.3
5 25 27 24 29
52
54 52b 53b 58 55 56 57
0.98 >2.5 >2.1 2.0 1.9 1.0 0.8
VOL. 32, NO. 4
2.2 >11.5 > 10.8 11.0 11.8 6.6 3.4
29 2.2 3.8 24.5 17.6 30 21.5
..
20 20 20 20 20 20 20
-StabilityRatioa
%
Tumbler Tests -HardnessRatio=
%
-Friability-
-Shatter Ratios
%
IndexRatioa
CORRESPONDING
-AgglutinatingValue Kg. Ratioa
..
YieldRatios
%
%
Oxidation Required t o Cause 20 Per Cent Decrease in Hardness of 20 0.77 53 0.8 78.7 1.09 59 28.5 0.84 49 0.8 74.8 1.09 75 47 0.79 67.2 55 0.8 1.26 83 55 59 0.78 55.5 0.8 1.48 85 0.78 56.5 59 53.5 0.8 1.45 89 47.5 52 0.76 62.7 0.8 1.32 83 48 51 0.78 54.0 1.14 0.8 88 .. Oxidation Required to Cause 10 Per Cent Decrease in Shatter Index 25 0.96 62.5 0.94 75.3 1.04 67
-Tar
Coke 0.80 0.98 0.94 0.90 0.93 0.88
1.6 6.1 4.9 3.4 3.2 3.8 4 2
0.62 0.73 0.60 0.64 0.48 0.74 0.68
6.8 6.4 5.1 2.8 2.7 1.8 1.7
0.91 0.66 0.63 0.72 0.64 0.75 0.85
of Coke 0.9
2.0
0.77
7.2
0.96
O:k4 0.64 0.39 0.78 0.60
419 2.8 2.6 1.85 1.65
0:60
n.. 02 _-
45 0:76 52:6 0:76 70:5 1:32 80 0:9 414 54 0.76 58.5 0.80 56.0 1.49 85 0.9 3.4 51 0.70 54 0.73 57.9 1.57 86 0.9 2.6 49 0.79 54 0.82 60.7 1.28 85 0.9 4.0 46 0.75 49 0.77 58.2 1.23 86 0.9 3.7 Oxidation Required t o Give 20 Per Cent Unfused Material on Carbonization 23 0.88 58 0.89 76.8 1.07 63 0.85 1.7
